1. Field of the Invention
This invention relates generally to microwave circulators, and more particularly to a thin-film microwave circulator.
2. Description of the Related Art
A microwave circulator is a passive multiple-port electronic device that transfers microwave energy in a non-reciprocal way, for example in a 3-port device energy entering into port 1 predominantly exits port 2, energy into port 2 exits port 3, and energy into port 3 exits port 1. The selection of ports is arbitrary, and circulators can be made to circulate either clockwise (CW) or counterclockwise (CCW). Microwave circulators may be used as part of an antenna interface in a transmit/receive system. Energy can be made to flow from the transmitter (port 1) to the antenna (port 2) during transmit, and from the antenna (port 2) to the receiver (port 3) during receive.
Microwave circulators may be implemented in a planar configuration using stripline or microstrip technology which employ a planar resonating ferrite element between two ground plane conductors (stripline) or coupled to a single ground plane conductor (microstrip). A slab of bulk ferrite material of appropriate dimensions is placed in the center region of the circulator and is magnetized generally perpendicular to the ground plane of the circulator by an external magnet. The magnet can be either a permanent magnet or an electromagnet. In the case of an electromagnet an additional current supply is needed to energize its coils. The magnetization of the ferrite slab can then be switched and the circulator mode of operation can be modified from CW to CCW by switching the magnetization of the ferrite slab. The ferrite material is chosen to have a ferromagnetic resonance (FMR) frequency that generally matches the operational frequency of the microwave signal, to thereby provide a non-reciprocal transmission path between ports.
Because of the requirement for the bulk ferrite material and the permanent magnet, microwave circulators are not compatible with integration into small devices that require compactness and light weight. Moreover, the ferrite material limits the operating frequency of the circulator to the resonance frequency of the ferrite. Thus what is needed is a tunable microwave circulator that is compatible with compact, light-weight thin film design and that can easily be integrated into small devices.